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aria-current="page" class="active sidebar-link">面试官：你有写过自定义指令吗？自定义指令的应用场景有哪些？</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/fe/vue/directive.html#一、什么是指令" class="sidebar-link">一、什么是指令</a></li><li class="sidebar-sub-header"><a href="/fe/vue/directive.html#三、应用场景" class="sidebar-link">三、应用场景</a></li><li class="sidebar-sub-header"><a href="/fe/vue/directive.html#参考文献" class="sidebar-link">参考文献</a></li></ul></li><li><a href="/fe/vue/filter.html" class="sidebar-link">面试官：Vue中的过滤器了解吗？过滤器的应用场景有哪些？</a></li><li><a href="/fe/vue/vnode.html" class="sidebar-link">虚拟DOM</a></li><li><a href="/fe/vue/diff.html" class="sidebar-link">Diff 算法</a></li><li><a href="/fe/vue/axios.html" class="sidebar-link">面试官：Vue项目中有封装过axios吗？主要是封装哪方面的？</a></li><li><a href="/fe/vue/axios_code.html" class="sidebar-link">面试官：你了解axios的原理吗？有看过它的源码吗？</a></li><li><a href="/fe/vue/ssr.html" class="sidebar-link">面试官：SSR解决了什么问题？有做过SSR吗？你是怎么做的？</a></li><li><a href="/fe/vue/structure.html" class="sidebar-link">面试官：说下你的vue项目的目录结构，如果是大型项目你该怎么划分结构和划分组件呢？</a></li><li><a href="/fe/vue/permission.html" class="sidebar-link">面试官：vue要做权限管理该怎么做？如果控制到按钮级别的权限怎么做？</a></li><li><a href="/fe/vue/cors.html" class="sidebar-link">面试官：Vue项目中你是如何解决跨域的呢？</a></li><li><a href="/fe/vue/404.html" class="sidebar-link">面试官：vue项目本地开发完成后部署到服务器后报404是什么原因呢？</a></li><li><a href="/fe/vue/error.html" class="sidebar-link">面试官：你是怎么处理vue项目中的错误的？</a></li><li><a href="/fe/vue/vue_router.html" class="sidebar-link">Vue Router</a></li><li><a href="/fe/vue/vue-debounce-throttle.html" class="sidebar-link">vue防抖调研</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Vue3</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>React</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Webpack</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Node.js</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>性能优化</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>手写代码</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="面试官-你有写过自定义指令吗-自定义指令的应用场景有哪些"><a href="#面试官-你有写过自定义指令吗-自定义指令的应用场景有哪些" class="header-anchor">#</a> 面试官：你有写过自定义指令吗？自定义指令的应用场景有哪些？</h1> <p><img src="https://static.vue-js.com/bd85a970-4345-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <h2 id="一、什么是指令"><a href="#一、什么是指令" class="header-anchor">#</a> 一、什么是指令</h2> <p>开始之前我们先学习一下指令系统这个词</p> <p><strong>指令系统</strong>是计算机硬件的语言系统，也叫机器语言，它是系统程序员看到的计算机的主要属性。因此指令系统表征了计算机的基本功能决定了机器所要求的能力</p> <p>在<code>vue</code>中提供了一套为数据驱动视图更为方便的操作，这些操作被称为指令系统</p> <p>我们看到的<code>v-</code>开头的行内属性，都是指令，不同的指令可以完成或实现不同的功能</p> <p>除了核心功能默认内置的指令 (<code>v-model</code> 和 <code>v-show</code>)，<code>Vue</code> 也允许注册自定义指令</p> <p>指令使用的几种方式：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token comment">//会实例化一个指令，但这个指令没有参数 </span>
<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">v-xxx</span><span class="token template-punctuation string">`</span></span>

<span class="token comment">// -- 将值传到指令中</span>
<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">v-xxx=&quot;value&quot;</span><span class="token template-punctuation string">`</span></span>  

<span class="token comment">// -- 将字符串传入到指令中，如`v-html=&quot;'&lt;p&gt;内容&lt;/p&gt;'&quot;`</span>
<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">v-xxx=&quot;'string'&quot;</span><span class="token template-punctuation string">`</span></span> 

<span class="token comment">// -- 传参数（`arg`），如`v-bind:class=&quot;className&quot;`</span>
<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">v-xxx:arg=&quot;value&quot;</span><span class="token template-punctuation string">`</span></span> 

<span class="token comment">// -- 使用修饰符（`modifier`）</span>
<span class="token template-string"><span class="token template-punctuation string">`</span><span class="token string">v-xxx:arg.modifier=&quot;value&quot;</span><span class="token template-punctuation string">`</span></span> 
</code></pre></div><h3 id="二、如何实现"><a href="#二、如何实现" class="header-anchor">#</a> 二、如何实现</h3> <p>注册一个自定义指令有全局注册与局部注册</p> <p>全局注册主要是通过<code>Vue.directive</code>方法进行注册</p> <p><code>Vue.directive</code>第一个参数是指令的名字（不需要写上<code>v-</code>前缀），第二个参数可以是对象数据，也可以是一个指令函数</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token comment">// 注册一个全局自定义指令 `v-focus`</span>
Vue<span class="token punctuation">.</span><span class="token function">directive</span><span class="token punctuation">(</span><span class="token string">'focus'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token comment">// 当被绑定的元素插入到 DOM 中时……</span>
  <span class="token function-variable function">inserted</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">el</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// 聚焦元素</span>
    el<span class="token punctuation">.</span><span class="token function">focus</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// 页面加载完成之后自动让输入框获取到焦点的小功能</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>局部注册通过在组件<code>options</code>选项中设置<code>directive</code>属性</p> <div class="language-js extra-class"><pre class="language-js"><code>directives<span class="token operator">:</span> <span class="token punctuation">{</span>
  focus<span class="token operator">:</span> <span class="token punctuation">{</span>
    <span class="token comment">// 指令的定义</span>
    <span class="token function-variable function">inserted</span><span class="token operator">:</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">el</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
      el<span class="token punctuation">.</span><span class="token function">focus</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 页面加载完成之后自动让输入框获取到焦点的小功能</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>然后你可以在模板中任何元素上使用新的 <code>v-focus</code> property，如下：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token operator">&lt;</span>input v<span class="token operator">-</span>focus <span class="token operator">/</span><span class="token operator">&gt;</span>
</code></pre></div><p>自定义指令也像组件那样存在钩子函数：</p> <ul><li><p><code>bind</code>：只调用一次，指令第一次绑定到元素时调用。在这里可以进行一次性的初始化设置</p></li> <li><p><code>inserted</code>：被绑定元素插入父节点时调用 (仅保证父节点存在，但不一定已被插入文档中)</p></li> <li><p><code>update</code>：所在组件的 <code>VNode</code> 更新时调用，但是可能发生在其子 <code>VNode</code> 更新之前。指令的值可能发生了改变，也可能没有。但是你可以通过比较更新前后的值来忽略不必要的模板更新</p></li> <li><p><code>componentUpdated</code>：指令所在组件的 <code>VNode</code> 及其子 <code>VNode</code> 全部更新后调用</p></li> <li><p><code>unbind</code>：只调用一次，指令与元素解绑时调用</p></li></ul> <p>所有的钩子函数的参数都有以下：</p> <ul><li><code>el</code>：指令所绑定的元素，可以用来直接操作 <code>DOM</code></li> <li><code>binding</code>：一个对象，包含以下 <code>property</code>：
<ul><li><code>name</code>：指令名，不包括 <code>v-</code> 前缀。</li> <li><code>value</code>：指令的绑定值，例如：<code>v-my-directive=&quot;1 + 1&quot;</code> 中，绑定值为 <code>2</code>。</li> <li><code>oldValue</code>：指令绑定的前一个值，仅在 <code>update</code> 和 <code>componentUpdated</code> 钩子中可用。无论值是否改变都可用。</li> <li><code>expression</code>：字符串形式的指令表达式。例如 <code>v-my-directive=&quot;1 + 1&quot;</code> 中，表达式为 <code>&quot;1 + 1&quot;</code>。</li> <li><code>arg</code>：传给指令的参数，可选。例如 <code>v-my-directive:foo</code> 中，参数为 <code>&quot;foo&quot;</code>。</li> <li><code>modifiers</code>：一个包含修饰符的对象。例如：<code>v-my-directive.foo.bar</code> 中，修饰符对象为 <code>{ foo: true, bar: true }</code></li></ul></li> <li><code>vnode</code>：<code>Vue</code> 编译生成的虚拟节点</li> <li><code>oldVnode</code>：上一个虚拟节点，仅在 <code>update</code> 和 <code>componentUpdated</code> 钩子中可用</li></ul> <blockquote><p>除了 <code>el</code> 之外，其它参数都应该是只读的，切勿进行修改。如果需要在钩子之间共享数据，建议通过元素的 <code>dataset</code> 来进行</p></blockquote> <p>举个例子：</p> <div class="language-html extra-class"><pre class="language-html"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>div</span> <span class="token attr-name">v-demo</span><span class="token attr-value"><span class="token punctuation attr-equals">=</span><span class="token punctuation">&quot;</span>{ color: <span class="token punctuation">'</span>white<span class="token punctuation">'</span>, text: <span class="token punctuation">'</span>hello!<span class="token punctuation">'</span> }<span class="token punctuation">&quot;</span></span><span class="token punctuation">&gt;</span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>div</span><span class="token punctuation">&gt;</span></span>
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>script</span><span class="token punctuation">&gt;</span></span><span class="token script"><span class="token language-javascript">
    Vue<span class="token punctuation">.</span><span class="token function">directive</span><span class="token punctuation">(</span><span class="token string">'demo'</span><span class="token punctuation">,</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">el<span class="token punctuation">,</span> binding</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>binding<span class="token punctuation">.</span>value<span class="token punctuation">.</span>color<span class="token punctuation">)</span> <span class="token comment">// &quot;white&quot;</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>binding<span class="token punctuation">.</span>value<span class="token punctuation">.</span>text<span class="token punctuation">)</span>  <span class="token comment">// &quot;hello!&quot;</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span>
</span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>script</span><span class="token punctuation">&gt;</span></span>
</code></pre></div><h2 id="三、应用场景"><a href="#三、应用场景" class="header-anchor">#</a> 三、应用场景</h2> <p>使用自定义指令可以满足我们日常一些场景，这里给出几个自定义指令的案例：</p> <ul><li>防抖</li> <li>图片懒加载</li> <li>一键 Copy的功能</li></ul> <h3 id="输入框防抖"><a href="#输入框防抖" class="header-anchor">#</a> 输入框防抖</h3> <p>防抖这种情况设置一个<code>v-throttle</code>自定义指令来实现</p> <p>举个例子：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token comment">// 1.设置v-throttle自定义指令</span>
Vue<span class="token punctuation">.</span><span class="token function">directive</span><span class="token punctuation">(</span><span class="token string">'throttle'</span><span class="token punctuation">,</span> <span class="token punctuation">{</span>
  <span class="token function-variable function">bind</span><span class="token operator">:</span> <span class="token punctuation">(</span><span class="token parameter">el<span class="token punctuation">,</span> binding</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> throttleTime <span class="token operator">=</span> binding<span class="token punctuation">.</span>value<span class="token punctuation">;</span> <span class="token comment">// 防抖时间</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>throttleTime<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 用户若不设置防抖时间，则默认2s</span>
      throttleTime <span class="token operator">=</span> <span class="token number">2000</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">let</span> cbFun<span class="token punctuation">;</span>
    el<span class="token punctuation">.</span><span class="token function">addEventListener</span><span class="token punctuation">(</span><span class="token string">'click'</span><span class="token punctuation">,</span> <span class="token parameter">event</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>cbFun<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 第一次执行</span>
        cbFun <span class="token operator">=</span> <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
          cbFun <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span><span class="token punctuation">,</span> throttleTime<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        event <span class="token operator">&amp;&amp;</span> event<span class="token punctuation">.</span><span class="token function">stopImmediatePropagation</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 2.为button标签设置v-throttle自定义指令</span>
<span class="token operator">&lt;</span>button @click<span class="token operator">=</span><span class="token string">&quot;sayHello&quot;</span> v<span class="token operator">-</span>throttle<span class="token operator">&gt;</span>提交<span class="token operator">&lt;</span><span class="token operator">/</span>button<span class="token operator">&gt;</span>
</code></pre></div><h3 id="图片懒加载"><a href="#图片懒加载" class="header-anchor">#</a> 图片懒加载</h3> <p>设置一个<code>v-lazy</code>自定义指令完成图片懒加载</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> LazyLoad <span class="token operator">=</span> <span class="token punctuation">{</span>
    <span class="token comment">// install方法</span>
    <span class="token function">install</span><span class="token punctuation">(</span><span class="token parameter">Vue<span class="token punctuation">,</span>options</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    	  <span class="token comment">// 代替图片的loading图</span>
        <span class="token keyword">let</span> defaultSrc <span class="token operator">=</span> options<span class="token punctuation">.</span>default<span class="token punctuation">;</span>
        Vue<span class="token punctuation">.</span><span class="token function">directive</span><span class="token punctuation">(</span><span class="token string">'lazy'</span><span class="token punctuation">,</span><span class="token punctuation">{</span>
            <span class="token function">bind</span><span class="token punctuation">(</span><span class="token parameter">el<span class="token punctuation">,</span>binding</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                LazyLoad<span class="token punctuation">.</span><span class="token function">init</span><span class="token punctuation">(</span>el<span class="token punctuation">,</span>binding<span class="token punctuation">.</span>value<span class="token punctuation">,</span>defaultSrc<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span><span class="token punctuation">,</span>
            <span class="token function">inserted</span><span class="token punctuation">(</span><span class="token parameter">el</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                <span class="token comment">// 兼容处理</span>
                <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token string">'IntersectionObserver'</span> <span class="token keyword">in</span> window<span class="token punctuation">)</span><span class="token punctuation">{</span>
                    LazyLoad<span class="token punctuation">.</span><span class="token function">observe</span><span class="token punctuation">(</span>el<span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
                    LazyLoad<span class="token punctuation">.</span><span class="token function">listenerScroll</span><span class="token punctuation">(</span>el<span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token punctuation">}</span>
                
            <span class="token punctuation">}</span><span class="token punctuation">,</span>
        <span class="token punctuation">}</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
    <span class="token comment">// 初始化</span>
    <span class="token function">init</span><span class="token punctuation">(</span><span class="token parameter">el<span class="token punctuation">,</span>val<span class="token punctuation">,</span>def</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token comment">// data-src 储存真实src</span>
        el<span class="token punctuation">.</span><span class="token function">setAttribute</span><span class="token punctuation">(</span><span class="token string">'data-src'</span><span class="token punctuation">,</span>val<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">// 设置src为loading图</span>
        el<span class="token punctuation">.</span><span class="token function">setAttribute</span><span class="token punctuation">(</span><span class="token string">'src'</span><span class="token punctuation">,</span>def<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
    <span class="token comment">// 利用IntersectionObserver监听el</span>
    <span class="token function">observe</span><span class="token punctuation">(</span><span class="token parameter">el</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">let</span> io <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">IntersectionObserver</span><span class="token punctuation">(</span><span class="token parameter">entries</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
            <span class="token keyword">let</span> realSrc <span class="token operator">=</span> el<span class="token punctuation">.</span>dataset<span class="token punctuation">.</span>src<span class="token punctuation">;</span>
            <span class="token keyword">if</span><span class="token punctuation">(</span>entries<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">.</span>isIntersecting<span class="token punctuation">)</span><span class="token punctuation">{</span>
                <span class="token keyword">if</span><span class="token punctuation">(</span>realSrc<span class="token punctuation">)</span><span class="token punctuation">{</span>
                    el<span class="token punctuation">.</span>src <span class="token operator">=</span> realSrc<span class="token punctuation">;</span>
                    el<span class="token punctuation">.</span><span class="token function">removeAttribute</span><span class="token punctuation">(</span><span class="token string">'data-src'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token punctuation">}</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        io<span class="token punctuation">.</span><span class="token function">observe</span><span class="token punctuation">(</span>el<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
    <span class="token comment">// 监听scroll事件</span>
    <span class="token function">listenerScroll</span><span class="token punctuation">(</span><span class="token parameter">el</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">let</span> handler <span class="token operator">=</span> LazyLoad<span class="token punctuation">.</span><span class="token function">throttle</span><span class="token punctuation">(</span>LazyLoad<span class="token punctuation">.</span>load<span class="token punctuation">,</span><span class="token number">300</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        LazyLoad<span class="token punctuation">.</span><span class="token function">load</span><span class="token punctuation">(</span>el<span class="token punctuation">)</span><span class="token punctuation">;</span>
        window<span class="token punctuation">.</span><span class="token function">addEventListener</span><span class="token punctuation">(</span><span class="token string">'scroll'</span><span class="token punctuation">,</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
            <span class="token function">handler</span><span class="token punctuation">(</span>el<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
    <span class="token comment">// 加载真实图片</span>
    <span class="token function">load</span><span class="token punctuation">(</span><span class="token parameter">el</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">let</span> windowHeight <span class="token operator">=</span> document<span class="token punctuation">.</span>documentElement<span class="token punctuation">.</span>clientHeight
        <span class="token keyword">let</span> elTop <span class="token operator">=</span> el<span class="token punctuation">.</span><span class="token function">getBoundingClientRect</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>top<span class="token punctuation">;</span>
        <span class="token keyword">let</span> elBtm <span class="token operator">=</span> el<span class="token punctuation">.</span><span class="token function">getBoundingClientRect</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span>bottom<span class="token punctuation">;</span>
        <span class="token keyword">let</span> realSrc <span class="token operator">=</span> el<span class="token punctuation">.</span>dataset<span class="token punctuation">.</span>src<span class="token punctuation">;</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span>elTop <span class="token operator">-</span> windowHeight<span class="token operator">&lt;</span><span class="token number">0</span><span class="token operator">&amp;&amp;</span>elBtm <span class="token operator">&gt;</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token keyword">if</span><span class="token punctuation">(</span>realSrc<span class="token punctuation">)</span><span class="token punctuation">{</span>
                el<span class="token punctuation">.</span>src <span class="token operator">=</span> realSrc<span class="token punctuation">;</span>
                el<span class="token punctuation">.</span><span class="token function">removeAttribute</span><span class="token punctuation">(</span><span class="token string">'data-src'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span><span class="token punctuation">,</span>
    <span class="token comment">// 节流</span>
    <span class="token function">throttle</span><span class="token punctuation">(</span><span class="token parameter">fn<span class="token punctuation">,</span>delay</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">let</span> timer<span class="token punctuation">;</span> 
        <span class="token keyword">let</span> prevTime<span class="token punctuation">;</span>
        <span class="token keyword">return</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token keyword">let</span> currTime <span class="token operator">=</span> Date<span class="token punctuation">.</span><span class="token function">now</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">let</span> context <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">;</span>
            <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">!</span>prevTime<span class="token punctuation">)</span> prevTime <span class="token operator">=</span> currTime<span class="token punctuation">;</span>
            <span class="token function">clearTimeout</span><span class="token punctuation">(</span>timer<span class="token punctuation">)</span><span class="token punctuation">;</span>
            
            <span class="token keyword">if</span><span class="token punctuation">(</span>currTime <span class="token operator">-</span> prevTime <span class="token operator">&gt;</span> delay<span class="token punctuation">)</span><span class="token punctuation">{</span>
                prevTime <span class="token operator">=</span> currTime<span class="token punctuation">;</span>
                <span class="token function">fn</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>context<span class="token punctuation">,</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token function">clearTimeout</span><span class="token punctuation">(</span>timer<span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token keyword">return</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>

            timer <span class="token operator">=</span> <span class="token function">setTimeout</span><span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                prevTime <span class="token operator">=</span> Date<span class="token punctuation">.</span><span class="token function">now</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                timer <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
                <span class="token function">fn</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span>context<span class="token punctuation">,</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span><span class="token punctuation">,</span>delay<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

<span class="token punctuation">}</span>
<span class="token keyword">export</span> <span class="token keyword">default</span> LazyLoad<span class="token punctuation">;</span>
</code></pre></div><h3 id="一键-copy的功能"><a href="#一键-copy的功能" class="header-anchor">#</a> 一键 Copy的功能</h3> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">import</span> <span class="token punctuation">{</span> Message <span class="token punctuation">}</span> <span class="token keyword">from</span> <span class="token string">'ant-design-vue'</span><span class="token punctuation">;</span>

<span class="token keyword">const</span> vCopy <span class="token operator">=</span> <span class="token punctuation">{</span> <span class="token comment">//</span>
  <span class="token comment">/*
    bind 钩子函数，第一次绑定时调用，可以在这里做初始化设置
    el: 作用的 dom 对象
    value: 传给指令的值，也就是我们要 copy 的值
  */</span>
  <span class="token function">bind</span><span class="token punctuation">(</span><span class="token parameter">el<span class="token punctuation">,</span> <span class="token punctuation">{</span> value <span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    el<span class="token punctuation">.</span>$value <span class="token operator">=</span> value<span class="token punctuation">;</span> <span class="token comment">// 用一个全局属性来存传进来的值，因为这个值在别的钩子函数里还会用到</span>
    el<span class="token punctuation">.</span><span class="token function-variable function">handler</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>el<span class="token punctuation">.</span>$value<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">// 值为空的时候，给出提示，我这里的提示是用的 ant-design-vue 的提示，你们随意</span>
        Message<span class="token punctuation">.</span><span class="token function">warning</span><span class="token punctuation">(</span><span class="token string">'无复制内容'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">return</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
      <span class="token comment">// 动态创建 textarea 标签</span>
      <span class="token keyword">const</span> textarea <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">createElement</span><span class="token punctuation">(</span><span class="token string">'textarea'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token comment">// 将该 textarea 设为 readonly 防止 iOS 下自动唤起键盘，同时将 textarea 移出可视区域</span>
      textarea<span class="token punctuation">.</span>readOnly <span class="token operator">=</span> <span class="token string">'readonly'</span><span class="token punctuation">;</span>
      textarea<span class="token punctuation">.</span>style<span class="token punctuation">.</span>position <span class="token operator">=</span> <span class="token string">'absolute'</span><span class="token punctuation">;</span>
      textarea<span class="token punctuation">.</span>style<span class="token punctuation">.</span>left <span class="token operator">=</span> <span class="token string">'-9999px'</span><span class="token punctuation">;</span>
      <span class="token comment">// 将要 copy 的值赋给 textarea 标签的 value 属性</span>
      textarea<span class="token punctuation">.</span>value <span class="token operator">=</span> el<span class="token punctuation">.</span>$value<span class="token punctuation">;</span>
      <span class="token comment">// 将 textarea 插入到 body 中</span>
      document<span class="token punctuation">.</span>body<span class="token punctuation">.</span><span class="token function">appendChild</span><span class="token punctuation">(</span>textarea<span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token comment">// 选中值并复制</span>
      textarea<span class="token punctuation">.</span><span class="token function">select</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token comment">// textarea.setSelectionRange(0, textarea.value.length);</span>
      <span class="token keyword">const</span> result <span class="token operator">=</span> document<span class="token punctuation">.</span><span class="token function">execCommand</span><span class="token punctuation">(</span><span class="token string">'Copy'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span>result<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        Message<span class="token punctuation">.</span><span class="token function">success</span><span class="token punctuation">(</span><span class="token string">'复制成功'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
      <span class="token punctuation">}</span>
      document<span class="token punctuation">.</span>body<span class="token punctuation">.</span><span class="token function">removeChild</span><span class="token punctuation">(</span>textarea<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token comment">// 绑定点击事件，就是所谓的一键 copy 啦</span>
    el<span class="token punctuation">.</span><span class="token function">addEventListener</span><span class="token punctuation">(</span><span class="token string">'click'</span><span class="token punctuation">,</span> el<span class="token punctuation">.</span>handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token comment">// 当传进来的值更新的时候触发</span>
  <span class="token function">componentUpdated</span><span class="token punctuation">(</span><span class="token parameter">el<span class="token punctuation">,</span> <span class="token punctuation">{</span> value <span class="token punctuation">}</span></span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    el<span class="token punctuation">.</span>$value <span class="token operator">=</span> value<span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
  <span class="token comment">// 指令与元素解绑的时候，移除事件绑定</span>
  <span class="token function">unbind</span><span class="token punctuation">(</span><span class="token parameter">el</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    el<span class="token punctuation">.</span><span class="token function">removeEventListener</span><span class="token punctuation">(</span><span class="token string">'click'</span><span class="token punctuation">,</span> el<span class="token punctuation">.</span>handler<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token keyword">export</span> <span class="token keyword">default</span> vCopy<span class="token punctuation">;</span>
</code></pre></div><p>关于自定义指令还有很多应用场景，如：拖拽指令、页面水印、权限校验等等应用场景</p> <h2 id="参考文献"><a href="#参考文献" class="header-anchor">#</a> 参考文献</h2> <ul><li>https://vue3js.cn/docs/zh</li> <li>https://juejin.cn/post/6844904197448531975#heading-5</li> <li>https://www.cnblogs.com/chenwenhao/p/11924161.html#_label2</li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">更新时间:</span> <span class="time">12/14/2021, 10:12:35 AM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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